Hitherto, alternating current losses have presented a serious problem in such cable systems. The losses arise from the fact that the a.c. resistance of electrical conductors increases when they are installed in a metallic enclosure. This increase is normally measured with reference to the a.c./d.c. resistance ratio. The various effects which contribute to the increases in a.c./d.c. ratio may be summarized as follows:
______________________________________ Designation Increase in a.c. Resistance due to: ______________________________________ Y.sub.cs skin effect in-air Y.sub.cp proximity effect in-air Y.sub.se shield eddy current effect in-air Y.sub.sc shield circulating current effect in-air Y'.sub.cs skin effect in-pipe Y'.sub.cp proximity effect in-pipe Y'.sub.se shield eddy current effect in-pipe Y'.sub.sc shield circulating current effect in-pipe Y.sub.p pipe loss effect. ______________________________________
In the case of a three-phase pipe-type cable system the a.c./d.c. resistance ratio is expressed by the equation ##EQU1## where the prime signs denote that the skin effect, proximity effect, shield eddy current and shield circulating current are affected by the presence of the pipe. These effects are considerably greater than in the case of a three-phase cable system in air.
It is important to note that the change in a.c./d.c. resistance ratio due to the metallic enclosure depends upon a large number of parameters related to the design and materials of the cables, the design and materials of the enclosure, the cable configuration and the operating conditions. Extensive work has been carried out by many researchers to establish methods for reducing the a.c. resistance and consequently the electrical losses in such systems. However, in spite of some progress in this area, the reported methods to reduce the electrical losses offer only partial solutions to the problem since they address only a limited number of the relevant parameters. As a result, none of the reported methods offers sufficient improvement to justify the cost of its implementation on a commercial scale.